1. Field of the Invention
The present invention relates to a non-aqueous electrolyte for a secondary battery and a non-aqueous electrolyte secondary battery employing such a non-aqueous electrolyte. More particularly, the present invention relates to an improved non-aqueous electrolyte which suppresses a reaction between an electrode and itself under high temperature and restricts decrease in battery capacity so that long time excellent battery characteristics can be obtained.
2. Description of the Related Art
In recent years, a non-aqueous electrolyte secondary battery, which employs a non-aqueous electrolyte wherein lithium ion is moved between a positive electrode and a negative electrode to perform charging and discharging, has been widely used as a new type of secondary battery featuring high power and high energy density.
Such a non-aqueous electrolyte secondary battery generally employs a non-aqueous electrolyte comprising a mixed solvent wherein cyclic carbonic acid ester, such as ethylene carbonate, and chain carbonic acid ester, such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate, are mixed, and lithium salt, such as LiPF6 or LiBF4, dissolved in the mixed solvent.
However, the non-aqueous electrolyte secondary battery using the above-described non-aqueous electrolyte has a problem as follows. When the non-aqueous electrolyte secondary battery is left in charging condition under high temperature for evaluating durability, a side reaction wherein the non-aqueous electrolyte reacts with the positive electrode and the negative electrode is caused, and battery capacity is decreased.
Therefore, it has disclosed that a variety of chain fluorinated carboxylic acid ester is used as a solvent of a non-aqueous electrolyte or as an additive to the non-aqueous electrolyte (See patent documents 1-6 below).    [Patent Document 1] JP-A 8-298134    [Patent Document 2] JP-A 11-86901    [Patent Document 3] JP-A 6-20719    [Patent Document 4] JP-A 2003-282138    [Patent Document 5] JP-A 2006-32300    [Patent Document 6] JP-A 2006-114388
Generally, if a non-aqueous solvent is fluorinated, oxidation-resistance of solvent is improved, and as a result, a reaction between a positive electrode and a non-aqueous electrolyte is suppressed. However, if the non-aqueous solvent is fluorinated, viscosity of the non-aqueous electrolyte is increased, and rise of reactivity with a negative electrode is caused because of lowering of reduction-resistance. Especially, the reactivity with negative electrode is greatly influenced by a position where fluorine is introduced.
However, in each patent document indicated as above, types of chain fluorinated carboxylic acid ester used are varied. As to a position of carbon where fluorine is introduced, the patent documents 1, 2 and 5 suggest it is preferable to replace hydrogen combined with α carbon with fluorine, and the patent documents 3 and 4 suggest that α carbon and any other carbon may be used.
Further, if chain fluorinated carboxylic acid ester wherein hydrogen combined with α carbon is replaced with fluorine, for example, ethyl trifluoroacetate CF3COOCH2CH3, is used as the non-aqueous solvent, lithium salt such as LiPF6 is not dissolved properly. If ethyl difluoroacetate CHF2COOCH2CH3 is used, although lithium salt such as LiPF6 is dissolved, the reactivity with the negative electrode is risen, and battery capacity is greatly decreased and battery characteristics are deteriorated in the case that the non-aqueous electrolyte secondary battery is left in charging condition under high temperature.
As a result, the use of chain fluorinated carboxylic acid ester wherein hydrogen combined with α carbon is replaced with fluorine can not attain sufficient battery characteristics.
Still further, if chain fluorinated carboxylic acid ester wherein hydrogen combined with carbon which is not α carbon is replaced with fluorine is used as the non-aqueous solvent of the non-aqueous electrolyte, the reactivity with the negative electrode is lowered. However, in the case that the non-aqueous electrolyte secondary battery is left in charging condition under high temperature, there remain problems that the battery capacity is decreased and the battery characteristics are deteriorated.
Also, when such a chain fluorinated carboxylic acid ester is used in combination of another non-aqueous solvent, if the non-aqueous solvent to be combined is not appropriate, initial capacity of the non-aqueous electrolyte secondary battery is decreased. Furthermore, in the case that the non-aqueous electrolyte secondary battery is left in charging condition under high temperature, the battery capacity and the battery characteristics are decreased.
As is described above, although fluorination of non-aqueous solvent contributes to suppression of the reaction between the positive electrode and itself, the reactivity between the negative electrode and itself is increased, so that excellent battery characteristics can not be obtained.